Implantable medical devices have been developed to support various body functions internally without having to connect wires through the skin. Examples of these devices include implantable pacemakers, cardioverters or defibrillators for monitoring and for stimulating ailing hearts. Implantable bladder stimulators provide electrical stimuli to bladder muscles to restore bladder function. Similar types of electrical stimuli provided by bone growth stimulators help the patients with complicated bone fractures. Cerebellar implantable devices monitor brain activities and stimulate the brain to control seizures in epilepsy as well as pain.
A common configuration of such implantable medical devices includes pulse generators with electronic circuitry contained in a housing and conductive leads that connect the pulse generators to the target organ or tissue. When the target organ or tissue also has a conductive path to the pulse generator, a conductive loop is established. Under normal usage of the device, no electric current flows through these conductive loops. When the patients implanted with the devices are subjected to external electromagnetic interference, however, undesirable electric current and voltage could be induced by such interference and could create undesirable physiological effects, such as fibrillation and pain.
Documented examples of medical device malfunctions have been traced to medical procedures, such as radiofrequency catheter ablation, electrocautary, dental procedures, magnetic resonance imaging (MRI) techniques, as well as other medical procedures. Of these well-known sources of electromagnetic interference, the MRI system is perhaps the most common.
MRI is a technique that provides a non-invasive method for the examination of the internal anatomy of a human body. This provides an efficient means for diagnosing disorders such as neurological and cardiac abnormalities. However, it may be unsafe and even hazardous to place patients implanted with the medical device through the MRI system because this may generate high radiofrequency (RF) signals, in the range of, for example, 64 MHz.
Such a high RF field may induce eddy currents on the conductive housing of the device, causing heat energy to be dissipated to the surrounding tissues, damaging them. More disadvantageously, the high RF field and resulting high induced voltage may demodulate the voltage within internal components of the implanted device. As a result, each RF pulse generated by the MRI system can generate voltage through the leads to the electrically excitable tissues.
In pacemakers or implantable cardioverter defibrillators (ICDs), the resulting high frequency output could, in certain cases, cause life-threatening fibrillation of the heart. Low energy voltage induced by the varying RF field may also inhibit the output of the pacing pulse to the patient. Such induction loops may exist in both the bipolar and unipolar lead configurations of the pacemaker or ICDs.
Several methods have been proposed to eliminate the effects of interference by the MRI systems on the implantable medical devices. In one such method a switching system provides high impedance to the conductive loop when a magnetic field above a preset threshold is detected. In another method an internal electronic circuitry is encased in a metal housing that is hermetically sealed to shield off external electromagnetic fields. Other methods propose using looped wires to offset stray RF fields and active RF filter to filter out a preselected range of RF frequency.
However, the foregoing methods provide only static isolation of undesirable interference from the implantable devices, and in some cases, the methods used to render the implanted device compatible with the operation of the MRI limit the ability of the implanted device to monitor external activities and to provide therapy at the preset time. Thus, it would be desirable to provide a dynamic approach that maximizes the magnetic and tuned RF detection response, while maintaining the integrity of the normal function of the implanted medical device. The need for such a design has heretofore remained unsatisfied.